from my own personal perspective and being a standard i now myself endeavour to meet one of the things i always found the
left its mark about an mmo with me is those golden treasure few individuals each MMO seems to have that when people begin to play an mmo anew, do not like the illiterate, immature masses on many scream noob at a persons ignorance over facets, and specifics, and tactics of a game, when they ask perfectly reasonable questions or advice.
Im speaking of those treasured few individuals who often spend a good few hours via roleplay or personal message helping and informing those so called inquisitive noobs and advising them to the best of thier abilities and not instead alienating them from a game straight away by being abusive.

There have been a few people across a number of mmo's ive played over the years that have met this criteria and really made my exposure to a particular new game memorable as a result. those people who are often nameless or forgotten just as quickly are the unsung heroes of MMO's who really and truely leave a mark worth commenting on.

just my opinion.

Although it wasn't exactly an MMO one of my characters once made a great impact on a Never Winter Nights Persistent World.

I role-played a chaotic religious fanatic
(some would call him evil, actually most would call him evil)
that worshiped a god of Chaos and Destruction, spreading his teachings to everyone he met and publicly sacrificing defeated enemies as well as worthless trinkets he might have found on the ground, to his devilish master.

At first players and GM's alike thought I was just another freak and payed little attention to me. But after a while my teachings started to sink in and before I knew it I had people asking if they could be my disciples.

I eventually left the game for other prospects but popped back in a few years later to see what had changed. I was surprised to see that not only did my legacy still thrive in the game, it had also become part of the games background story.

The world had statues, books and other references to my god, religion and cult. There were even npcs and a few quests revolving around my legacy.
And although the members of my old cult had all forgotten about their prophet, they were still spreading my teachings and still proudly wore my guild emblem on their cloaks.

Even now, 5-6 years later, the stories of evil are told to young children in the Three Kingdoms.

"Blood for the Blood God!" - Kalrach, the Prophet.

//
The Three Kingdoms(3kds) is the name of the NWN Persistent World.
Kalrach was the name of my character in 3kds.

I'm rather modest, when it comes to this kind of stuff, but I can say I've left something of an impact in some of the MMOs I've played. :)

I was part of the development of the first ever DKP system sorta. Rather I was part of the raid lead by soon to be Afterlife members on Lady Vox in EQ on Mithaniel Marr server that made everyone realize randoming for items just wouldn't work in the long run. A druid won the roll on the uber dru/rng scimitar with the pet proc which lead to some drama and Thott and a few others coming up with the first DKP system. Poor rangers Tanthalar, Dekein and Dulorn(?) =(

aww the good ole days

Vaul - EQ Paladin Mithaniel Marr/Drinal servers